This invention relates to the production of high impact strength molding compositions obtained by thermal treatment of synthetic resin mixtures containing predominantly polyalkylene terephthalate, in the solid form, as well as to the thus-obtained molding compositions as such.
Polyalkylene terephthalates are valuable building materials having a number of excellent properties, such as high rigidity, surface hardness, abrasion resistance, a relatively high deflection temperature under load, dimensional stability, and rapid processability to complicated and large-size molded articles.
Conversely, the impact strength of the polyalkylene terephthalates is inadequate for some applications, especially at temperatures of below 20.degree. C., so that there is a need to develop polyalkylene terephthalate molding compositions having higher impact strengths. By "impact strength" in connection with this invention is meant that measured by using a notched bar as is conventional in the plastics industry. In the United States, a conventional procedure is the Izod test (ASTM D256) and in the Federal Republic of Germany DIN 53 453.
Numerous suggestions have been advanced for improving the low temperature impact strength of polyalkylene terephthalate molding compositions by the incorporation of aliphatic dicarboxylic acids or diol mixtures by condensation, or by blending with other polymers, such as modified elastomers and polyolefins. However, all of these measures have the drawback that either the increase in low temperature impact strength is only minor or that a marked improvement in low temperature impact strength is accompanied by an undesirable impairment of other properties, especially rigidity and deflection temperature under load.
One group of elastomers proposed for modifying the impact strength of polyalkylene terephthalates comprises alkyl acrylate multistage graft copolymers. Thus, U.S. Pat. No. 4,034,013 and DOS No. 2,726,256 describe the use of multistage graft copolymers of an elastomeric basic polymer and a graft component. The basic polymers are preferably derived from butyl acrylate while methacrylates, preferably methyl methacrylates, are utilized as the graft component.
DOS No. 3,012,333 describes impact resistant compositions of polyalkylene terephthalates and polyacrylate graft copolymers; the blends can be additionally reinforced by glass fibers and/or can be made flame-retardant.
Impact resistant injection molding compositions of polyalkylene terephthalates, acrylate multistage graft copolymers, polyether ester elastomers, and specific stabilizer combinations are described in European Pat. Nos. 33,393 and 32,586.
However, impact strength improving effect of the polyacrylate additives in the blends with polyalkylene terephthalates is unsatisfactory insofar as high proportions of modifier are required to obtain a relatively minor increase in the impact strength. Due to the high elastomer proportion, rigidity and deflection temperature under load of the polyester molding compositions are markedly impaired.
Likewise, with respect to another group of modifiers, DOS No. 2,930,343 describes molding compositions based on polyalkylene terephthalate with contain blended-in polyether ester amides, in order to improve impact strength, but here again, such relatively high proportions of the polyether ester amide must be incorporated into the polyalkylene terephthalate to achieve an increased impact strength, that rigidity and deflection temperature under load of the polyester are clearly impaired.
In still another group of modifiers, copolymers of .alpha.-olefins, the same problem exists. For details, in DOS No. 1,694,168, there is described the use of 0.05-20% by weight--based on the total mixture--of copolymers of .alpha.-olefins and vinyl esters of saturated aliphatic monocarboxylic acids for the modification of polyethylene terephthalate molding compositions. DOS's Nos. 2,756,698 and 3,040,999 and U.S. Pat. No. 4,101,502 claim reinforced polyalkylene terephthalate molding compositions blended with ethylene/vinyl acetate copolymers to improve toughness, and impact-resistant molding compositions of polyethylene terephthalate and/or polybutylene terephthalate and copolymers of .alpha.-olefins and esters of acrylic acid are described, for example, in DOS's Nos. 1,694,170 and 2,248,242.